1. Technical Field
The present invention generally relates to high-frequency filters used in, for example, the wireless communication field, and more particularly to a dual-mode filter generating two different resonant modes by introducing electromagnetic discontinuity in a ring resonator, and a tuning method of such a dual-mode filter.
2. Background Art
Recently, with prevalence and development of cell phones, fast and high-capacity transmission technologies have becomes indispensable. To realize such a fast and high-capacity transmission technology, a wide frequency band is required. Therefore, the frequency range used in wireless communications is being shifted to a higher frequency range. As a result, the filters used for wireless communications are need to have characteristics capable of selectively passing a desired communication frequency and cutting off the frequency components other than the desired pass band even in a high frequency range. Furthermore, there is strong demand for the sizes and weights of such wireless communication apparatuses using high frequency circuit elements to be further reduced.
A ring resonator is known as a resonant filter including a ring having a circumference of one wavelength or an integral number of wavelengths as an electrical path length. To improve the space efficiency of the ring resonator, methods of generating two resonant modes (a dual-mode resonator) within a single resonator and obtaining sharp filter characteristics are disclosed in Patent Documents 1 and 2.
Patent Document 1: Japanese Patent No. 3304724
Patent Document 2: Japanese Patent Application Publication No. 2000-209002
Patent Document 1 discloses a dual-mode filter including a ring resonator, where an input line and an output line are coupled to the ring resonator orthogonal to each other. A stub is provided in between a coupling point of the input line and the output line with respect to the ring resonator. Due to the stub, an electromagnetic discontinuity point (or perturbation) is generated, thereby generating a dual-mode resonator. However, disadvantageously, current concentration occurs in the vicinity of the discontinuity point in the ring of the resonator, which may degrade the power durability of the resonator.
Further, Patent Document 2 discloses a dual-mode filter including a ring resonator, where an input line and an output line are coupled to the ring resonator orthogonal to each other. A distributed coupling line is provided at the position on a median line equidistant from the coupling points of the input and the output lines and the resonator along the outer circumference of the ring of the resonator. Due to the distributed coupling line, a dual-mode ring resonator is generated. However, disadvantageously, the position at which the distributed coupling line can be disposed is limited to the point on the median line equidistant from the coupling points, thereby reducing the design degree of freedom for the position of the distributed coupling line.